Here you go, a Short Guide to Bitcoin Hardforks.

The overview is that there has been an ongoing debate (argument) among developers of how Bitcoin should scale (or if it should scale) to accommodate greater transaction volume and greater transaction sizes. There have been multiple proposals. The only way for a new proposal to become active is for the developers to coordinate a date and time at which miners and node operators who agree with the changes to the code to decide to point their mining and transaction confirming activity to the new code, essentially a new blockchain.

To ensure that people who own Bitcoin have the option to adopt the new version of the coin they will give you coins in the new version of the coins equal to the amount of Bitcoin you own at the time of the hard fork. One important stipulation is that if you don't own your private keys to your coins you can't guarantee you'll have access to the new currency. Some exchanges are offering to provide their users the new coins, but not all.

Exchanges that honored payment of Bitcoin Cash during the August 1 hard fork were Bittrex, Freebitco.in, and very few others. Coinbase, for example, was not going to offer the new coins but decided to after public outcry after the fork.